Modular arithmetic example

mod.js

// JS remainder operator `%` won't work for our case,
// we need a modulo that follows modular arithmetics definition.
function mod(num, modulus) {
  return ((num % modulus) + modulus) % modulus;
}

// Alternative (not strict, but IMO easier to remember)
function modAlt(num, modulus) {
  const x = num % modulus;
  return x < 0 ? x + modulus : x;
}

/* Example usage */

class Playlist {
  #songs;
  #index = 0;

  constructor(songs) {
    this.#songs = songs;
  }

  get current() {
    return this.#songs[this.#index];
  }

  offsetBy(num) {
    this.#index = mod(this.#index + num, this.#songs.length);
    return this.current;
  }

  next() {
    return this.offsetBy(1);
  }

  prev() {
    return this.offsetBy(-1);
  }
}

const playlist = new Playlist(["song_1.mp3", "song_2.mp3", "song_3.mp3"]);

console.log(playlist.current);      // song_1.mp3
console.log(playlist.offsetBy(2));  // song_3.mp3
console.log(playlist.next());       // song_1.mp3
console.log(playlist.prev());       // song_3.mp3